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Michaela Fecková, Pascal Le Poul, Françoise Robin-Le Guen, Thierry Roisnel, Oldřich Pytela, Milan Klikar, Filip Bureš, Sylvain Achelle
To cite this version:
Michaela Fecková, Pascal Le Poul, Françoise Robin-Le Guen, Thierry Roisnel, Oldřich Pytela, et al.. 2,4-Distyryl- and 2,4,6-Tristyrylpyrimidines Synthesis and Photophysical Properties.
Journal of Organic Chemistry, American Chemical Society, 2018, 83 (19), pp.11712-11726.
�10.1021/acs.joc.8b01653�. �hal-01880133�
2,4-Distyryl- and 2,4,6-tristyrylpyrimidines: Synthesis and photophysical properties.
Michaela Fecková,†, ‡ Pascal le Poul,† Françoise Robin-le Guen,† Thierry Roisnel,† Oldřich Pytela,‡ Milan Klikar, ‡ Filip Bureš,*,‡ and Sylvain Achelle*,†
† Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, F- 35000 Rennes, France. ‡ Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice, 53210, Czech Republic.
Corresponding authors: *E-mails: sylvain.achelle@univ-rennes1.fr; filip.bures@upce.cz
Synopsis TOC
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Abstract 3
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The synthesis of a series of twenty new 2,4,6-tristyrylpyrimidines and three new 2,4- distyrylpyrimidines by means of combination of Knoevenagel condensation and Suzuki- Miyaura cross coupling reaction is reported. This methodology enables to obtain chromophores with identical or different substituent on each arm. The photophysical properties of the compounds are described. Optical properties and TD-DFT calculations indicate that photophysical properties of target compounds are mainly affected by the nature of the electron-donating group in C4/C6 positions, except when the C2 substituent is a significantly stronger electron-donating group. However, the C2 substituent has a strong influence on emission quantum yield: addition of a strong electron-donating group tends to decrease the fluorescence quantum yield whereas a moderate electron-withdrawing group results in a significant increase of fluorescence quantum yield.
Introduction
During the past two decades, there has been a great interest in the synthesis of pyrimidine fluorophores.1 The pyrimidine is a six-membered heterocycle with two nitrogen atoms (1,3-diazine) that exhibits a strong electron-withdrawing character. When the pyrimidine ring is combined with electron-donating fragments via π-conjugated linkers, intramolecular charge transfer (ICT) occurs leading, generally, to a strong emission.1 Recently, pyrimidine-based thermally activated fluorescent emitters (TADF) have been developed and have been used for high external quantum efficiency organic light-emitting diodes (OLEDs).2 4,6-Distyrylpyrimidines have also been developed as two-photon excitation emitters for biological microscopy3 and photoinitiator for multiphoton lithography.4
Due to the electron lone pair of its two nitrogen atoms, the pyrimidine ring can catch protons, coordinate metal cations, and link to various (bio)organic molecules leading to modification of its emission properties. Various pyrimidine chromophores have been used as pH,5 metal 3
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cations,6 nitroaromatic7, and protein8 fluorescent sensors. Mixtures of neutral and protonated forms of pyrimidine fluorophores have also been used as white light emitters.9 Recently, Kato and coworkers have highlighted the influence of substituent in C2 position on the emission properties of pyrimidine derivatives.10
Y-shaped centripetal molecules have been subject to intensive research.11 In this context, the triazine core has been extensively used. 12 In these structures, periphery-to-core multidimensional charge transfer lead to large second-order optical response13 and large two photon absorption cross section. 14 Numerous 2,4,6-triaryl- and 2,4,6- triarylethnynylpyrimidine chromophores have been designed for their luminescence properties.15,16
Styrylpyrimidines are generally obtained by Knoevenagel condensation of an aldehyde with methylpyrimidine under basic or acidic conditions.4,5,8,9a,10, 17
This reaction can be easily carried out on the methyl group in positions C2, C4 and/or C6. Using this strategy, 2-styryl,18 4-styryl3c,9a,17 and 4,6-distrylpyrimidine3,4,5a,8,10,17
as well as 4,4',6,6'-tetrastyrylpyrimidine19 chromophores have been prepared so far; selected known 4,6-distyrylpyrimidines are presented in Chart 1. To the best of our knowledge, only the unsubstituted 2,4,6- tristyrylpyrimidine has been described to date.20
Chart 1. Structures of selected known chromophores 4,6-distyrylpyrimidines.5a,17a
In this contribution, we describe the design and synthesis of 2,4,6-tristyrylpyrimidines and 2,4-distyrylpyrimidines by combining Suzuki-Miyaura cross coupling reaction and Knoevenagel condensation. This strategy enables the synthesis of compounds with different substituents on each arm and, thus, allows modification of their electron donating or withdrawing abilities. The molecular structure obtained by X-ray analysis of a selected 3
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chromophore is also described. The photophysical properties of target chromophores were studied and thoroughly compared with the corresponding 4,6-distyrylpyrimidines. The DFT and TD-DFT calculations were also performed on selected chromophores to rationalize their photophysical properties.
Results and discussion Synthesis
Various strategies have been considered for the synthesis of 2,4,6-tristyrylpyrimidines.
Threefold Knoevenagel condensation from 2,4,6-trimethylpyrimidine has been envisioned.
However, the pyrimidine starting material cannot be isolated easily in a good yield.21 The synthesis of 2-methyl-4,6-distyrylpyrimidine was also considered by treating 4,6- distyrylpyrimidine with methyllithium followed by in-situ rearomatization with DDQ according to known procedure22 but we failed in obtaining the desired product. We therefore proposed a third strategy, starting from commercially available 2-chloro-4,6- dimethylpyrimidine and 2,4-dichloro-6-methylpyrimidine. A combination of Suzuki cross- coupling and Knoevenagel condensation has been developed leading to twenty-one tristyrylpyrimidine chromophores (Schemes 1–2).
Chromophores 2 with identical substituents in C4 and C6 positions were obtained in two steps from 2-chloro-4,6-dimethylpyrimidine. The first step consists of the in-situ conversion of arylalkynes into styrylboronic acid by action of catecholborane followed by palladium- catalyzed Suzuki-Miyaura cross coupling reaction.23 2-Styrylpyrimidine intermediates 1 were obtained in moderate to good yields. 2,4,6-Tristyrylpyrimidines 2 were obtained by condensation between the C4 and C6 methyl group and the corresponding aromatic aldehyde in boiling aqueous 5 M NaOH using Aliquat 336 as catalyst.5a,17a Moderate to good yields 3
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were generally observed. The lower yields, observed in particular for unsubstituted styryl derivatives, were due to more complicated purification process.
Scheme 1: Synthesis of compounds 2.
To obtain 2,4,6-tristyrylpyrimidines 5 with three different arms, a three-step synthetic route employed 2,4-dichloro-6-methylpyrimidine as a starting material (Scheme 2). The first step consisted in a C4 regioselective Suzuki-Miyaura cross coupling reaction with in situ formed styrylboronic acid leading to intermediates 3. The observed regioselectivity of cross coupling reaction in C4 over C2 position of the pyrimidine ring is in accordance with the literature.15b,16a, 24
The second step consisted in a second Suzuki-Miyaura cross coupling reaction in C2 position leading to 2,4-distyrylpyrimidines intermediates 4 and finally a Knoevenagel condensation of 4-( N,N-diphenylamino)benzaldehyde on the methyl group in C6 position lead to chromophores 5.
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Scheme 2: Synthesis of compounds 5.
Starting from 2-chloro-4-methylpyrimidine, 2,4-distyrylpyrimidines were obtained in a similar two-step synthetic pathway (Scheme 3).
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Scheme 3: Synthesis of compounds 7.
All new compounds are well soluble especially in chlorinated solvents (DCM, CHCl3) and were characterized by 1H, 13C NMR and HRMS spectroscopic techniques. The selectivity of the Suzuki-Miyaura cross coupling as well as condensation reactions was sufficiently high to generate all trans-isomer within the limits of NMR detection as observed previously.5a The stereochemistry of the double bounds was unequivocally established on the basis of coupling constant for the vinylic proton in the 1H NMR (J ≈ 16 Hz). No trans/cis isomerisation were observed during photophysical experiments.
X-ray Analysis
Chromophore 2h provided crystals for X-ray analysis by slow evaporation of its dichloromethane/ethyl acetate (1/1, v/v) solution. The measured crystal, a large fragment of a large yellow and transparent prism, confirms the proposed molecular structure; in particular the E configuration of the three vinylic linkers is confirmed (Figure 1). An orthorhombic crystal system is observed with a Pna21 space group. The presence of a (modelled) disorder 3
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on one of the two C4/C6 arm, already observed for similar structures,19b can be shown. Solid state supramolecular assembly of the chromophore revealed an orthorhombic crystal system with a Pna21 space group. The crystal structure shows that the angles between the planes of three benzene rings and the pyrimidine central core lower than 20°, indicating that they are not completely planar, in accordance with other 4-styryl- and 4,6-distyrylpyrimidines.9a,25 It should be noted that the dihedral angle between the phenyl ring in C2 position and the pyrimidine core is the lowest (∼7°). Bond length alternation (BLA) were calculated for the vinylic linker on each arm and were revealed to be slightly lower for the C2 arm (0.1185 Ǻ), than for C4/C6 arms (0.1205, 0.1600 and 0.1775 Ǻ) indicating that the C2 arm imparts stronger ICT.
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Figure 1: ORTEP drawing of the chromophore 2h with thermal ellipsoid at 50%.
Thermal properties
Thermal behaviour of the final compounds 2, 5 and 7 as well as compounds A2 and A3
was studied by differential scanning calorimetry (DSC). Figure 2 shows thermograms of representative compounds 2i, 2m, A2 and A3 while Table 1 lists all measured melting points (Tm) and temperatures of thermal decompositions (Td). All DCS curves are given in the SI.
The measured melting points range from 128 to 243 °C. The temperature of decomposition was estimated within the range of 195-320 °C. Further discussion on the thermal properties is provided in the SI.
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Figure 2: Representative DSC thermographs of compounds 2i, 2m, A2 and A3 obtained with a scanning rate of 3°C/min in the range 50-400°C
Table 1: DSC results for chromophores 2, 5, 7, A2 and A3.
Comp. Tm (°C)
Td
(°C) Comp. Tm (°C)
Td
(°C) Comp. Tm (°C)
Td
(°C) Comp. Tm (°C)
Td (°C) 2a 193 300 2h 200 295 2o / 250 7a 137 290
2b 128 290 2i / 250 2p / 220 7b 185 305
2c / 245 2j / 240 2q / 280 7c 243 265
2d / 305 2k / 310 2r 210 265 7d / 250
2e 183 305 2l / 195 2s / 300 A2 191 260
2f / 250 2m / 310 5a / 240 A3 205 320
2g / 270 2n / 290 5b / 210
Tm = melting point (the point of intersection of a baseline and a tangent of thermal effect = onset). Td = thermal decomposition (pyrolysis in N2 atmosphere).
Common feature of tripodal (D-π)3-A or (D-π)2-A-(π-A) chromophores 2a–s and 5a–b is their resistance to crystallize and they often occur as solid glasses and amorphous solid.
This suggests a more sophisticated thermal behaviour of centripetal star-shaped systems.
Hence, their DSC curves are frequently similar and decorated by solid-solid or glass transitions. Based on the DSC analysis there are no evident general trends between type or position of particular donor used and thermal behavior of tripodal chromophores. Structural 3
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modifications (donor variation) often influences the thermal properties of tripodal chromophores only negligibly because the intrinsic nature of these molecules remains the same.
In general, 2,4- and 4,6-disubstituted pyrimidines (7a–d, A2 and A3) have demonstrated a similar thermal behaviour. A presence of donor groups (NMe2, OMe) in quadrupolar molecules 7b and 7c increases the melting point up to 100 °C compared to
“donor free” analogue 7a.
The Td of tripodal molecules 2i/2m and their quadrupolar analogous A2/A3 are comparable (Figure 2) and dictated by the type of attached donors on the periphery.
Therefore, DMA 2i/A2 and TPA analogous 2m/A3 have demonstrated almost identical Td
values (250/260 and 310/320 °C, see Table 1).
Photophysical properties
The UV/Vis and photoluminescence (PL) spectroscopic data of compounds 2, 5 and 7 measured in CH2Cl2 at room temperature are presented in Table 2. The analyses were carried out by using low concentrations of chromophores (0.5–1.5 × 10-5 M). To facilitate comparison of photophysical properties, 4,6-distyrylpyrimidines A5a were also included in Table 1. As an example, the spectra of compounds 2b, 2c and 2d are provided in Figure 3.
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Figure 3: Normalized absorption (solid lines) and emission spectra (dashed lines) of compounds 2b (black), 2c (green) and 2d (green) in CH2Cl2 solution.
For all the compounds, the less energetic absorption band (λmax = 312-429 nm) is attributed to charge transfer. Except compounds 2a, 2h, 7a and 7b, that are however slightly emissive in non polar heptane, all the compounds exhibit significant emission. In case of 2h, the absence of emission is attributed to twisted intramolecular charge transfer excited state. For these compounds, as generally observed for pyrimidine push-pull chromophores,1 large Stokes shifts were obtained, indicating large difference (vibrational, electronic, geometric) between the Franck-Condon state and the excited state. For tristyrylpyrimidines 2 and 5, the position of absorption and emission wavelength maxima depends mainly on C4/C6 substituents.
However, it should be noted that a significant red-shift in emission is observed when the C2 3
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substituent is a significantly stronger electron-donating groups than C4/C6 substituents (2j and 2k). Whereas the ICT generally occurs over the C4/C6 arm, in this case, the ICT is probably prevailing on the C2 arm. The fluorescence quantum yield varies significantly when the C2 substituent is modified. When tristrylpyrimidines 2b-2d with unsubstituted styryl group in C2 position are compared with 2,6-distyrylpyrimidines A1-A3, it appears that the addition of the styryl group in C2 position leads to a slight red shift of the emission maxima and a significant increase of the fluorescence quantum yield. The presence of an electron- donating group (methoxy, dimethylamino or diphenylamino) on the C2 arm (compounds 2e- 2g, 2i, 2l, 2m) results in a slight blue shift of emission regarding the corresponding unsubstituted styryl derivatives 2b-2d. The opposite trend is observed for 2j and 2k probably due to change in the ICT direction over the C2 arm. Whereas chromophores with dimethylamino group on C4/C6 arms (compounds A2, 2c, 2f, 2l, 2o, 2r and 5a) are generally highly luminescent, the incorporation of this fragment on C2 (chromophores 2h-2j, 5b and 7c) leads to a dramatic decrease of fluorescence quantum yield. On the other hand, the addition of a trifluoromethyl fragment, a moderately electron-withdrawing group, on C2 arm (chromophores 2q-2s) results in a significant increase of the fluorescence quantum yield up to 1.00 for 2s. 2,4-Distyrylpyrimidines 7 exhibits significantly blue shifted absorption and emission with regards to their 4,6-distyryl and 2,4,6-tristyrylpyrimidines analogues. All these trends are in accordance with the observation made on arylpyrimidine series.1a,15a-b
Table 2: UV/Vis and PL data in CH2Cl2.
Compda
UV/vis λmax, nm (ε, mM–1·cm–1)
PL
λmax, nm ΦFb
Stokes shift cm–1
A117a 359 (36.0) 439 - 5076
A25a 429 (42.1) 530 0.40 4442
A35a 427 (47.6) 540 0.55 4901
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2a 312 (74.3) – – –
2b 369 (35.9) 442 0.10 4476
2c 423 (48.1) 537 0.72 5019
2d 430 (55.0) 551 0.78 5107
2e 339 (64.0) 439 0.15 6719
2f 423 (45.6) 533 0.53 4879
2g 424 (68.9) 543 0.91 5168
2h 370 (94.0) – – –
2i 402 (97.3) 536 0.22 6219
2j 409 (112.9) 577 0.03 7118
2k 376 (61.7) 558 0.10 7799
2l 406 (42.4) 533 0.59 5869
2m 415 (86.2) 544 0.56 5714
2n 347 (61.6) 443 0.11 6245
2o 418 (52.5) 536 0.52 5266
2p 424 (69.7) 544 0.75 5202
2q 370 (26.8) 441 0.21 4351
2r 429 (49.1) 545 0.72 4961
2s 430 (60.4) 554 1.00 5205
5a 420 (40.4) 549 0.68 5595
5b 386 (58.1) 548 0.02 7659
7a 305 (41.3) – – –
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7b 331 (53.9) – – –
7c 395 (67.2) 493 0.04 5032
7d 402 (45.5) 527 0.72 5900
a All spectra were recorded at room temperature at c = 0.5 × 10–5 M to 1.5 × 10–5. b Fluorescence quantum yield (±10%) determined relative to 9,10-bisphenylethynylanthracene in cyclohexane (ΦF = 1.00).26
In order to gain further insights into the photophysical properties of these compounds, their absorption and emission behavior was studied in a variety of aprotic solvents. While the absorption maxima were not significantly shifted, an increase in solvent polarity, estimated by Dimroth-Reichardt polarity parameter,27 led to bathochromic shifts of the emission maxima.
The results of emission solvatochromism are summarized in Table 3. As an example, the spectra registered for compound 2g are shown in Figure 4 and the change in emission color under UV irradiation for compound 2k, easily seen by the naked eye, can be seen in Figure 5.
This solvatochromic behavior, characteristic for fluorophore featuring intramolecular charge transfer, can be explained by the stabilization of the highly polar emittting state by polar solvents.28 For all the compounds, the emission maxima were plotted versus Dimroth- Reichardt polarity parameter (see Figures S106-S112) and, in all cases, a good linearity was observed. The emission solvatochromic behavior can be quantified by the slope of the corresponding regression line. Once again, with the exception of compound 2k, the substituents in C4/C6 arm are the most affected by the emission solvatochromism with decreasing order: NMe2 > NPh2 > OMe. This is in accordance to the electron-donating strength of the substituents. The slopes for 2,4-distyrylpyrimidines 7c and 7d are significantly lower than that of their 2,4,6-tristyrylpyrimidine analogues 2i and 2m, indicating that both arms in C4 and C6 position play a role on the emission solvatochromism and, therefore on ICT into the chromophores.
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Table 3: Emission solvatochromism of pyrimidine derivatives in various aprotic solvents.
Heptane Toluene THF CH2Cl2 Acetone MeCN DMSO
Compd ET(30)a = 30.9 ET(30)a = 33.9 ET(30)a = 37.4 ∆ET(30)a = 40.7 ∆ET(30)a = 42.2 ∆ET(30)a = 45.6 ∆ET(30)a = 45.1 λmax, nm λmax, nm λmax, nm λmax, nm λmax, nm λmax, nm λmax, nm
2b 397, 420 408, 429 432 442 450 470 471
2c 453 483 526 537 563 587 601
2d 464 482 520 551 563 591 596
2e 402, 421 410, 433 432 439 440 453 461
2f 450 480 522 533 559 581 598
2g 461 480 515 543 555 583 584
2i 450 479 521 536 558 577 601
2j 460 495 512 547 552 574 577
2k 439 466 511 558 572 627 622
2l 451 480 521 533 561 584 596
2m 460 479 514 544 555 589 586
2n 401, 422 411, 433 433 443 445 463 471
2o 449 480 520 536 565 586 597
2p 460 479 519 544 558 586 585
2q 398 411 436 441 452 463 475
2r 456 486 533 545 565 587 592
2s 467 486 526 554 567 600 599
5a 457 491 542 549 571 594 589
5b 398 411 436 441 452 463 475
7c 433 455 484 493 523 513 527
7d 447 470 502 527 531 546 547
a Dimroth–Reichardt polarity parameter, kcal⋅mol–1 3
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Figure 4: Normalized emission spectra of 2g in different aprotic solvents.
Figure 5: Fluorescence color changes experienced by 2k in various solvents. Picture was taken in the dark upon irradiation with a hand-held UV lamp (λem = 366 nm).
The ability of protonation of pyrimidine chromophores has been already demonstrated.5,9 Generally, the protonation of amino-substituted stryrylpyrimidines results in quenching of emission; however, in the case of less electron-donating substituents, such as methoxy groups, 3
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the protonation lead to a red-shifted emission.5a,9a The changes in the emission spectra of 2q upon addition of (1S)-(+)-10-camphorsulfonic acid are illustrated in Figure 6. The progressive disappearance of the emission band of the neutral form at 441 nm is observed, whereas a new red-shifted absorption band at 540 nm progressively appear corresponding to the monoprotonated species.9a As shown on Figure 7, the neutral form of 2q emits dark blue light under UV-irradiation whereas the protonated form emits green light. A mixture of the two forms enables to obtain cyan light.
Figure 6 Changes in the emission spectra of a CH2Cl2 solution of 2q (c = 0.9642 ×10-5 M) upon addition of (1S)-(+)-10-camphorsulfonic acid (0.1 – 50 eq.). λexc = 380 nm.
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Figure 7 Change in the color of a CH2Cl2 solution of 2q (c = 0.9642 ×10-5 M) after the addition of 0.8 equiv (middle) and 50 equiv of (1S)-(+)-10-camphorsulfonic acid (right).
Picture was taken in the dark upon irradiation with a hand-held UV lamp (λem = 366 nm).
DFT calculations
DFT calculations implemented in Gaussian 16 software package29 were employed to investigate the fundamental properties of representative pyrimidines A3, 2g, 2j, 2k, and 2m with a systematically varied structural arrangement. Their optimized geometries, the HOMO/LUMO energies, and ground state dipole moments µ (Table 4) were obtained by DFT B3LYP/6-311+g(2d,p) method. The calculated HOMO/LUMO levels were further visualized in the energy level diagram as shown in Figure 8. Whereas the HOMO-LUMO gaps of all molecules are almost identical (2.97-3.02 eV), the principal variations are seen at both HOMO/LUMO levels. Hence, decoration of the central pyrimidine acceptor with two or three donor arms affects the position of the HOMO and LUMO rather than the HOMO-LUMO difference. This is in accordance to our latest observation and generalization on multipodal chromophores.30 The frontier molecular orbitals are further visualized in Figure 9. In the parent molecule A3, the HOMO is localized on both N,N-diphenylamino donors but is also 3
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mixed with the LUMO spread over the central pyrimidine and adjacent π-linkers. This implies lower charge separation. The situation is similar in 2g bearing weak additional 4-methoxystyryl donor appended at C2 (the second N,N-diphenylamino donor is occupied by the HOMO-1). However, by attaching strong electron releasing moieties, such as N,N- dimethylamino or N,N-diphenylamino groups, the HOMO has completely moved on the arms appended at C2 regardless what type of donors are connected at C4/C6. Hence, for 2j, 2k, and 2m, the ICT dominates from the arm connected at pyrimidine C2.
Table 4: DFT calculated data of representative chromophores.
Compda
EHOMO
eV
ELUMO
eV
∆E eV
µ D
λmax
nm/eV
A3 –5.26 –2.27 2.99 3.39 475/2.61
2g –5.21 –2.19 3.02 2.00 468/2.65
2j –5.09 –2.12 2.97 1.41 458/2.71
2k –5.14 –2.13 3.01 3.89 403/3.08
2m –5.14 –2.21 2.94 1.60 464/2.67
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Figure 8 Energy level diagram of the DFT-derived HOMO/LUMO energies.
Figure 9 Optimized geometries and HOMO (red) and LUMO (blue) localizations in representative chromophores.
The electronic absorption spectra of representative chromophores were further calculated using TD-SCF (nstates = 10) B3LYP/6-311+g(2d,p) method. The spectra are listed in Figure S113 jointly with the experimental data; the calculated longest-wavelength absorption maxima are given in Table 3. Their correlation with the experimental λmax values is very tight 3
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as shown in Figure S114. This implies that, despite the calculated values are generally red- shifted, the employed method is capable to described trends within the given series of molecules. The spectra of representative chromophores feature a low energy lying CT-band with the λmax appearing between 403-475 nm (376-428 nm experimentally). Compared to tripodal molecules 2, the quadrupolar chromophore A3 showed the most bathochromically shifted CT-band (λmax = 428 nm). This may be explained by the Frenkel exciton model, which predicts splitting of the singlet excited state into two and three bands for quadrupolar and tripodal molecules, as compared to their linear analogue. Whereas for quadrupolar molecules is the low energy band dominant, the low energy lying two states of a tripodal molecule are degenerate while the third high energy state has zero oscillator strength. Hence, the absorption spectra of both types of molecules are characterized by a single CT-band, eventually accompanied by a shoulder for quadrupolar molecules and a blue-shift for tripodal ones.
According to the performed calculations, the CT-bands of quadrupolar A3 and tripodal 2g, bearing weak methoxy electron donor, are almost exclusively generated by the HOMO→LUMO transition. On the other hand, the CT bands of 2j and 2m, bearing all amino donors, are dominated by the HOMO→LUMO, HOMO-1→LUMO, and HOMO→LUMO+1 transitions. Replacement of two amino donors at C4/C6 as in 2k led to a significantly red- shifted HOMO→LUMO transition (~468 nm, see Figure S113) but with diminished oscillator strength. Hence, the observed CT-band is generated by the HOMO→LUMO+1 and HOMO- 1→LUMO transitions.
Conclusions
In conclusion, we have successfully synthesized 2,4,6-tristyrylpyrimidine and 2,4- distyrylpyrimidine chromophores with identical or different substituent on each arm. In terms of absorption and emission maxima, and calculated HOMO-LUMO gaps, 2,4,6- tristyrylpyrimidines exhibits generally the similar properties as the corresponding 4,6- 3
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distyrylpyrimidines. Nevertheless, when the C2 substituent is a significantly stronger electron-donating substituents than C4/C6 ones, the ICT occurs mainly on this branch and dictates the photophysical properties. In all cases, the C2 substituent play a key role on the emission quantum yield. As one would expected, all these materials exhibit strong emission solvatochromism and pH sensibility. Comparison of the two-photon absorption properties of 2,4,6-tristyrylpyrimidine with the corresponding 4,6-distyrylpyrimidines are currently under investigation.
Experimental Section
General Conditions. In air- and moisture-sensitive reactions, all glassware was flame-dried and cooled under nitrogen. Thermal behavior of the target compounds was measured in open aluminous crucibles under N2 inert atmosphere. DSC curves were determined with a scanning rate of 3 °C/min within the range 25–400 °C. NMR spectra were acquired at room temperature. Chemical shifts are given in parts per million relative to TMS (1H, 0.0 ppm) and CDCl3 (13C, 77.0 ppm). Acidic impurities in CDCl3 were removed by treatment with anhydrous K2CO3. High resolution MALDI MS spectra were measured on a MALDI mass spectrometer equipped with nitrogen UV laser (337 nm, 60 Hz) and quadrupole analyser (positive-ion mode over a normal mass range (m/z 50-2000) with resolution 100 000 at m/z = 400). Trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was used as a matrix. Mass spectra were averaged over the whole MS record for all measured samples. UV-vis and fluorescence spectra were recorded using standard 1 cm quartz cells. Compounds were excited at their absorption maxima (band of lowest energy) to record the emission spectra. The ΦF values were calculated using a well-known procedure with 9,10-diphenylethynylanthracene in cyclohexane as standard.26 Stokes shifts were calculated by considering the lowest energetic absorption band. All calculations were carried out in Gaussian 09W package at the DFT level of theory. The initial geometry optimizations were carried out by the PM3 method implemented in program ArgusLab and subsequently by the DFT B3LYP method using the 6-311G++(2d,f,p) basic 3
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set. The energies of the HOMO and LUMO (EHOMO and ELUMO), their differences (∆E) and ground state dipole moments (µ) were calculated by the DFT B3LYP/6-311++G(2d,f,p) method.
General procedure for Suzuki-Miyaura cross-coupling reaction. The corresponding acetylene (1.57 equiv) was dissolved in THF (20 mL) and nitrogen was bubbled through the solution for 10 min. Catecholborane (1.9 equiv of 1 M solution in THF) was added and the reaction mixture was heated to reflux for 1.5 h. The second portion of catecholborane (0.7 equiv) was added and heating was continued for 2 h. The reaction mixture was cooled to room temperature and Pd(PPh3)4 (0.025 equiv) and corresponding pyrimidine (1 equiv) were added.
Solution was stirred for 20 min before 20% aqueous Na2CO3 (5 mL) was added and the mixture was stirred under nitrogen at reflux for 15 h. The reaction mixture was cooled and then diluted with CH2Cl2 (20 mL). The organic layer was washed with water (3x20 mL), then with brine (20 mL), separated, dried over MgSO4 and the solvents were evaporated under reduced pressure. The crude product was purified by column chromatography (SiO2, indicated solvents).
General procedure for Knoevenagel condensation. Aldehyde (1 equiv) and corresponding 4-methylpyrimidine (1 equiv) or 4,6-dimethylpyrimidine (0.5 equiv) were added in 5 M aqueous NaOH (15 mL) containing Aliquat 336 (0.1 equiv). Solution was heated to reflux for 3 h and then cooled to room temperature. The precipitate was filtered off, washed with water, and purified by recrystallization from CH2Cl2/n-heptane and/or by column chromatography (SiO2, indicated solvents).
(E)-2-Styryl-4,6-dimethylpyrimidine (1a). Synthesized from phenylacetylene (161 mg, 1.57 mmol) and 2-chloro-4,6-dimethylpyrimidine (143 mg, 1 mmol) following the general procedure for Suzuki-Miyaura reaction. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 7:3). Yield: 78 mg (37 %); white solid. Rf: 0.7 (SiO2; petroleum ether:EtOAc, 7:3). Mp: 47.5–49.4 °C (lit.31 47-50°C). 1H NMR (300 3
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MHz, CDCl3): δ = 2.50 (s, 6H), 6.86 (s, 1H), 7.21 (d, 3J = 15.9 Hz, 1H), 7.29–7.41 (m, 3H), 7.61–7.64 (m, 2H), 7.97 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 24.1, 117.7, 127.7, 127.9, 128.8, 128.9, 136.4, 137.6, 164.3, 166.6 ppm. IR (ATR): ν = 3057, 1533, 1367, 978, 747, 692 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C14H15N2 [(M+H)+] 211.1230, found 211.1227.
(E)-2-(4-Methoxystyryl)-4,6-dimethylpyrimidine (1b). Synthesized from 4-ethynylanisole (415 mg, 3.14 mmol) and 2-chloro-4,6-dimethylpyrimidine (286 mg, 2 mmol) following the general procedure for Suzuki-Miyaura reaction. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 7:3). Yield: 257 mg (54 %); brownish solid.
Rf: 0.4 (SiO2; petroleum ether:EtOAc, 7:3). Mp: 87.9–90.2 °C. 1H NMR (300 MHz, CDCl3):
δ = 2.48 (s, 6H), 3.82 (s, 3H), 6.82 (s, 1H), 6.89–6.92 (m, 2H), 7.07 (d, 3J = 15.9 Hz, 1H), 7.54–7.57 (m, 2H), 7.92 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 24.1, 55.4, 114.3, 117.4, 125.6, 129.1, 129.2, 137.3, 160.4, 164.6, 166.6 ppm. IR (ATR): ν = 3004, 1574, 1511, 1242, 1178, 1028, 981, 839, 818, 770 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C15H16N2O [M+] 240.1257, found 240.1256.
(E)-2-(4-Dimethylaminostyryl)-4,6-dimethylpyrimidine (1c). Synthesized from 4-ethynyl- N,N-dimethylaniline (362 mg, 2.50 mmol) and 2-chloro-4,6-dimethylpyrimidine (230 mg, 1.61 mmol) following the general procedure for Suzuki-Miyaura reaction. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 7:3). Yield: 244 mg (59 %); yellow solid. Rf: 0.6 (SiO2; petroleum ether:EtOAc, 7:3). Mp: 112.8–115.2 °C. 1H NMR (300 MHz, CDCl3): δ = 2.47 (s, 6H), 3.00 (s, 6H), 6.69–6.72 (m, 2H), 6.78 (s, 1H), 7.01 (d, 3J = 15.9 Hz, 1H), 7.51–7.54 (m, 2H), 7.91 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 24.2, 40.4, 112.2, 116.9, 123.2, 124.6, 129.1, 138.0, 151.0, 165.1, 166.4 ppm. IR (ATR): ν = 2919, 1580, 1443, 1356, 1165, 986, 820, 748 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C16H19N3 [M+] 253.1574, found 253.1562.
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(E)-2-(4-Diphenylaminostyryl)-4,6-dimethylpyrimidine (1d). Synthesized from 4-ethynyl- N,N-diphenylaniline (790 mg, 2.94 mmol) and 2-chloro-4,6-dimethylpyrimidine (267 mg, 1.87 mmol) following the general procedure for Suzuki-Miyaura reaction. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 8:2). Yield: 200 mg (28 %); yellow solid. Rf: 0.4 (SiO2; petroleum ether:EtOAc, 8:2). Mp: 132.7–135.7 °C. 1H NMR (300 MHz, CDCl3): δ = 2.49 (s, 6H), 6.83 (s, 1H), 7.02–7.14 (m, 9H), 7.25–7.30 (m, 4H), 7.46–7.49 (m, 2H), 7.91 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 24.1, 117.3, 122.6, 123.5, 125.1, 125.6, 128.6, 129.4, 129.9, 137.3, 147.4, 148.6, 164.6, 166.5 ppm. IR (ATR): ν = 3030, 1585, 1488, 1273, 749, 692 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C26H23N3 [M+] 377.1887, found 377.1888.
(E)-2-[(6-Methoxynaphtalen-2-yl)ethenyl]-4,6-dimethylpyrimidine (1e). Synthesized from 2-ethynyl-6-methoxynaphtalene (286 mg, 1.57 mmol) and 2-chloro-4,6-dimethylpyrimidine (143 mg, 1 mmol) following the general procedure for Suzuki-Miyaura reaction. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 7:3). Yield:
230 mg (79 %); yellowish solid. Rf: 0.3 (SiO2; petroleum ether:EtOAc, 7:3). Mp: 133.5–134.9
°C. 1H NMR (300 MHz, CDCl3): δ = 2.45 (s, 6H), 3.86 (s, 3H), 6.75 (s, 1H), 7.06–7.13 (m, 2H), 7.26 (d, 3J = 15.9 Hz, 1H), 7.66–7.77 (m, 3H), 7.88 (s, 1H), 8.09 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 24.0, 55.3, 105.9, 117.4, 119.1, 124.4, 127.0, 127.3, 128.4, 128.9, 129.9, 131.7, 134.9, 137.7, 158.2, 164.3, 166.4 ppm. IR (ATR): ν = 2931, 1582, 1364, 1163, 1027, 860, 803, 665 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C19H18N2O [M+] 290.1414, found 290.1414.
(E)-2-(4-Trifluoromethylstyryl)-4,6-dimethylpyrimidine (1f). Synthesized from 4- trifluoromethylphenylacetylene (426 mg, 2.50 mmol) and 2-chloro-4,6-dimethylpyrimidine (228 mg, 1.59 mmol) following the general procedure for Suzuki-Miyaura reaction. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 7:3). Yield:
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222 mg (50 %); yellowish solid. Rf: 0.4 (SiO2; petroleum ether:EtOAc, 7:3). Mp: 94.9–96.5
°C. 1H NMR (300 MHz, CDCl3): δ = 2.50 (s, 6H), 6.89 (s, 1H), 7.26 (d, 3J = 15.9 Hz, 1H), 7.60–7.71 (m, 4H), 7.97 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 24.1, 118.2, 124.2 (q, 1JCF = 270 Hz), 125.8 (q, 3JCF = 4 Hz), 127.7, 130.4, 130.5 (q, 2JCF = 32 Hz), 135.8, 139.9 (d, 4JCF = 1 Hz), 163.7, 166.8 ppm. IR (ATR): ν = 2929, 1583, 1319, 1103, 1064, 831, 714 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C15H14F3N2 [(M+H)+] 279.1104, found 279.1102.
(E,E,E)-2,4,6-Tristyrylpyrimidine (2a). Synthesized from 1a (300 mg, 1.43 mmol) and benzaldehyde (303 mg, 2.85 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 9:1) and then by recrystallization from CH2Cl2/n-heptane. Yield: 155 mg (28
%); white solid. Rf: 0.4 (SiO2; petroleum ether:EtOAc, 9:1). Mp: 193 °C (lit.20b 197-199°C).
1H NMR (300 MHz, CDCl3): δ = 7.08–7.16 (m, 3H), 7.28–7.43 (m, 10H), 7.62–7.69 (m, 6H), 7.93 (d, 3J = 15.9 Hz, 2H), 8.11 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 113.7, 126.5, 127.7, 127.8, 128.3, 128.8, 128.9, 129.3, 136.0, 136.4, 136.7, 137.9, 162.9, 164.6 ppm All the atoms of carbon were not observed.. IR (ATR): ν = 3025, 1635, 1564, 1514, 1368, 963, 739, 689 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C28H23N2
[(M+H)+] 387.1856, found 387.1855.
(E,E,E)-2-Styryl-4,6-bis(4-methoxystyryl)pyrimidine (2b). Synthesized from 1a (78 mg, 0.37 mmol) and 4-methoxybenzaldehyde (102 mg, 0.74 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 8:2) and then by recrystallization from CH2Cl2/n-heptane. Yield: 40 mg (24 %); yellowish solid. Rf: 0.4 (SiO2; petroleum ether:EtOAc, 8:2). Mp: 128 °C. 1H NMR (300 MHz, CDCl3): δ = 3.86 (s, 6H), 6.93–7.01 (m, 6H), 7.11 (s, 1H), 7.29–7.44 (m, 4H), 7.58–7.61 (m, 4H), 7.68–7.70 (m, 2H), 7.89 (d, 3J = 3
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15.9 Hz, 2H), 8.11 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 55.5, 113.2, 114.5, 124.4, 127.8, 128.5, 128.9, 128.90, 128.91, 129.2, 136.2, 136.6, 137.6, 160.7, 163.1, 164.5 ppm. IR (ATR): ν = 2837, 1604, 1560, 1501, 1251, 1171, 1152, 967, 749 cm-1. HR- MALDI-MS (DCTB): m/z calculated for C30H27N2O2 [(M+H)+] 447.2067, found 447.2064.
(E,E,E)-2-Styryl-4,6-bis(4-dimethylaminostyryl)pyrimidine (2c). Synthesized from 1a (150 mg, 0.71 mmol) and 4-N,N-dimethylaminobenzaldehyde (213 mg, 1.43 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 8:2) and then by recrystallization from CH2Cl2/n-heptane. Yield: 63 mg (17 %); black solid. Rf: 0.2 (SiO2; petroleum ether:EtOAc, 8:2). Td: 245 °C. 1H NMR (300 MHz, CDCl3): δ = 3.03 (s, 12 H), 6.71–6.74 (m, 4H), 6.91 (d, 3J = 15.9 Hz, 2H), 7.08 (s, 1H), 7.28–7.35 (m, 4H), 7.53–7.56 (m, 4H), 7.68–
7.70 (m, 2H), 7.86 (d, 3J = 15.9 Hz, 2H), 8.10 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.4, 112.2, 112.4, 114.2, 114.7, 121.9, 124.3, 127.8, 128.7, 128.8, 129.2, 136.8, 137.2, 151.2, 163.4, 164.3 ppm. IR (ATR): ν = 2892, 1602, 1555, 1503, 1363, 1162, 965, 800, 746 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C32H32N4 [M+] 472.2622, found 472.2620.
(E,E,E)-2-Styryl-4,6-bis(4-diphenylaminostyryl)pyrimidine (2d). Synthesized from 1a (300 mg, 1.43 mmol) and 4-N,N-diphenylaminobenzaldehyde (780 mg, 2.85 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 9:1) and then by recrystallization from CH2Cl2/n-heptane. Yield: 220 mg (21 %); yellow solid. Rf: 0.4 (SiO2; petroleum ether:EtOAc, 9:1). Td: 305 °C. 1H NMR (300 MHz, CDCl3): δ = 6.98 (d, 3J = 15.9 Hz, 2H), 7.05–7.17 (m, 17H), 7.27–7.43 (m, 12H), 7.48–7.51 (m, 4H), 7.67–7.70 (m, 2H), 7.87 (d, 3J = 15.9 Hz, 2H), 8.10 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 113.1, 122.5, 123.8, 124.4, 125.2, 127.8, 128.6, 128.77, 128.85, 128.88, 129.56, 129.59, 136.1, 136.6, 3
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137.6, 147.3, 149.0, 163.1, 164.6 ppm. IR (ATR): ν = 3033, 1559, 1490, 1274, 969, 748, 693 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C52H40N4 [M+] 720.3248, found 720.3252.
(E,E,E)-2,4,6-Tris(4-methoxystyryl)pyrimidine (2e). Synthesized from 1b (100 mg, 0.42 mmol) and 4-methoxybenzaldehyde (114 mg, 0.83 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 156 mg (79 %); yellowish solid. Rf: 0.4 (SiO2; petroleum ether:EtOAc, 8:2). Mp: 183 °C. 1H NMR (300 MHz, CDCl3): δ = 3.85 (s, 9H), 6.93–7.00 (m, 8H), 7.08 (s, 1H), 7.18 (d, 3J = 15.9 Hz, 1H), 7.57–7.65 (m, 6H), 7.88 (d, 3J = 15.9 Hz, 2H), 8.06 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 55.46, 55.49, 112.8, 114.34, 114.4, 124.5, 126.3, 129.0, 129.2, 129.4, 136.1, 137.3, 160.4, 160.7, 163.1, 164.8 ppm All the atoms of carbon were not observed.. IR (ATR): ν = 2933, 1602, 1559, 1506, 1245, 1169, 1028, 959, 868, 810, 767 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C31H28N2O3
[M+] 476.2094, found 476.2094.
(E,E,E)-2-(4-Methoxystyryl)-4,6-bis(4-dimethylaminostyryl)pyrimidine (2f). Synthesized from 1b (100 mg, 0.42 mmol) and 4-N,N-dimethylaminobenzaldehyde (125 mg, 0.83 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 154 mg (74 %); red solid. Rf: 0.3 (SiO2; petroleum ether:EtOAc, 8:2). Td: 250 °C. 1H NMR (300 MHz, CDCl3): δ = 3.03 (s, 12H), 3.85 (s, 3H), 6.71–6.74 (m, 4H), 6.88–6.95 (m, 4H), 7.07 (s, 1H), 7.17 (d, 3J = 15.9 Hz, 1H), 7.52–7.55 (m, 4H), 7.62–7.65 (m, 2H), 7.84 (d, 3J = 15.9 Hz, 2H), 8.05 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.4, 55.4, 112.1, 112.2, 114.3, 122.0, 124.3, 126.6, 129.1, 129.6, 136.6, 136.8, 151.1, 160.2, 163.3, 164.7 ppm All the atoms of carbon were not observed.. IR (ATR): ν = 2924, 1600, 1550, 1497, 1355, 1244, 1142, 971, 806 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C33H34N4O [M+] 502.2727, found 502.2731.
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(E,E,E)-2-(4-Methoxystyryl)-4,6-bis(4-diphenylaminostyryl)pyrimidine (2g). Synthesized from 1b (100 mg, 0.42 mmol) and 4-N,N-diphenylaminobenzaldehyde (228 mg, 0.83 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 225 mg (72 %); yellow solid. Rf: 0.7 (SiO2; petroleum ether:EtOAc, 8:2). Td: 270 °C. 1H NMR (300 MHz, CDCl3): δ = 3.85 (s, 3H), 6.92–7.21 (m, 24H), 7.27–7.32 (m, 6H), 7.48–7.50 (m, 4H), 7.61–7.64 (m, 2H), 7.86 (d,
3J = 15.9 Hz, 2H), 8.06 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 55.5, 112.8, 114.3, 122.6, 123.7, 124.5, 125.2, 126.3, 128.7, 129.2, 129.4, 129.5, 129.7, 136.0, 137.2, 147.4, 149.0, 160.4, 163.0, 164.9 ppm. IR (ATR): ν = 3033, 1562, 1489, 1271, 1242, 1168, 972, 818, 749, 691 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C53H42N4O [M+] 750.3353, found 750.3361.
(E,E,E)-2-(4-Dimethylaminostyryl)-4,6-bis(4-methoxystyryl)pyrimidine (2h). Synthesized from 1c (110 mg, 0.43 mmol) and 4-methoxybenzaldehyde (119 mg, 0.87 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 121 mg (57 %); brown solid. Rf: 0.3 (SiO2; petroleum ether:EtOAc, 8:2). Mp: 200 °C. 1H NMR (300 MHz, CDCl3): δ = 3.02 (s, 6H), 3.85 (s, 6H), 6.72–6.75 (m, 2H), 6.93–7.00 (m, 6H), 7.05 (s, 1H), 7.12 (d, 3J = 15.9 Hz, 1H), 7.57–
7.61 (m, 6H), 7.87 (d, 3J = 15.9 Hz, 2H), 8.05 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.4, 55.5, 112.2, 112.4, 114.4, 123.7, 124.7, 124.8, 129.0, 129.2, 135.9, 138.1, 151.0, 160.6, 163.0, 165.4 ppm All the atoms of carbon were not observed. IR (ATR): ν = 2924, 1603, 1558, 1502, 1357, 1249, 1168, 1028, 961, 810 cm-1. HR-MALDI-MS (DCTB):
m/z calculated for C32H31N3O2 [M+] 489.2411, found 489.2414.
(E,E,E)-2,4,6-Tris(4-dimethylaminostyryl)pyrimidine (2i). Synthesized from 1c (244 mg, 0.96 mmol) and 4-N,N-dimethylaminobenzaldehyde (287 mg, 1.92 mmol) following the 3
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general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 212 mg (43 %); brown solid. Rf: 0.1 (SiO2; petroleum ether:EtOAc, 8:2). Td: 250 °C. 1H NMR (300 MHz, CDCl3): δ = 3.02–3.03 (m, 18H), 6.71–6.75 (m, 6H), 6.91 (d, 3J = 15.9 Hz, 2H), 7.05 (s, 1H), 7.11 (d, 3J = 15.6 Hz, 1H), 7.53–7.61 (m, 6H), 7.83 (d, 3J = 15.9 Hz, 2H), 8.03 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.4, 40.5, 111.6, 112.3, 122.3, 124.2, 124.4, 125.0, 129.1, 136.4, 137.6, 150.9, 151.1, 163.2, 165.2 ppm All the atoms of carbon were not observed.. IR (ATR): ν = 2888, 1599, 1502, 1356, 1167, 965, 797 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C34H37N5 [M+] 515.3044, found 515.3044.
(E,E,E)-2-(4-Dimethylaminostyryl)-4,6-bis(4-diphenylaminostyryl)pyrimidine (2j).
Synthesized from 1c (110 mg, 0.43 mmol) and 4-N,N-diphenylaminobenzaldehyde (238 mg, 0.87 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 225 mg (68 %);
yellow solid. Rf: 0.6 (SiO2; petroleum ether:EtOAc, 8:2). Td: 240 °C. 1H NMR (300 MHz, CDCl3): δ = 3.02 (s, 6H), 6.72–6.75 (m, 2H), 6.99 (d, 3J = 15.9 Hz, 2H), 7.05–7.16 (m, 18H), 7.27–7.32 (m, 8H), 7.48–7.50 (m, 4H), 7.57–7.60 (m, 2H), 7.85 (d, 3J = 15.9 Hz, 2H), 8.04 (d,
3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.4, 112.2, 112.3, 122.6, 123.7, 124.76, 124.78, 125.2, 128.7, 129.2, 129.5, 129.8, 135.8, 138.0, 147.4, 148.9, 151.0, 163.0, 165.4 ppm All the atoms of carbon were not observed. IR (ATR): ν = 3034, 1564, 1490, 1361, 1273, 1167, 971, 810, 749, 692 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C54H45N5 [M+] 763.3670, found 763.3673.
(E,E,E)-2-(4-Diphenylaminostyryl)-4,6-bis(4-methoxystyryl)pyrimidine (2k). Synthesized from 1d (70 mg, 0.19 mmol) and 4-methoxybenzaldehyde (51 mg, 0.37 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 8:2). Yield: 83 mg (73 %); yellow solid. Rf: 3
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0.5 (SiO2; petroleum ether:EtOAc, 8:2). Td: 310 °C. 1H NMR (300 MHz, CDCl3): δ = 3.86 (s, 6H), 6.93–7.20 (m, 16H), 7.27–7.32 (m, 4H), 7.53–7.60 (m, 6H), 7.87 (d, 3J = 15.9 Hz, 2H), 8.05 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 55.5, 112.8, 114.4, 122.8, 123.6, 124.5, 126.4, 128.8, 128.9, 129.2, 129.5, 130.3, 136.1, 137.2, 147.5, 148.6, 160.7, 163.1, 164.9 ppm. IR (ATR): ν = 3033, 1562, 1506, 1279, 1248, 1031, 972, 808, 752, 695 cm-
1. HR-MALDI-MS (DCTB): m/z calculated for C42H35N3O2 [M+] 613.2724, found 613.2728.
(E,E,E)-2-(4-Diphenylaminostyryl)-4,6-bis(4-dimethylaminostyryl)pyrimidine (2l).
Synthesized from 1d (70 mg, 0.19 mmol) and 4-N,N-dimethylaminobenzaldehyde (56 mg, 0.37 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (SiO2, petroleum ether:EtOAc, 8:2) and then by recrystallization from CH2Cl2/n-heptane. Yield: 62 mg (52 %); yellow solid. Rf: 0.5 (SiO2; petroleum ether:EtOAc, 8:2). Td: 195 °C. 1H NMR (300 MHz, CDCl3): δ = 3.03 (s, 12H), 6.71–6.74 (m, 4H), 6.90 (d, 3J = 15.9 Hz, 2H), 7.03–7.08 (m, 5H), 7.13–7.19 (m, 5H), 7.28–
7.31 (m, 4H), 7.52–7.55 (m, 6H), 7.83 (d, 3J = 15.9 Hz, 2H), 8.03 (d, 3J = 15.9 Hz, 1H) ppm.
13C NMR (75 MHz, CDCl3): δ = 40.4, 112.0, 112.2, 122.1, 122.9, 123.5, 124.3, 125.1, 126.8, 128.7, 129.2, 129.5, 130.5, 136.6, 136.8, 147.5, 148.4, 151.2, 163.3, 164.7 ppm. IR (ATR): ν
= 2853, 1602, 1553, 1492, 1358, 1276, 974, 808, 751, 696 cm-1. HR-MALDI-MS (DCTB):
m/z calculated for C44H41N5 [M+] 639.3357, found 639.3359.
(E,E,E)-2,4,6-Tris(4-diphenylaminostyryl)pyrimidine (2m). Synthesized from 1d (33 mg, 0.09 mmol) and 4-N,N-diphenylaminobenzaldehyde (48 mg, 0.17 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 44 mg (57 %); yellow solid. Rf: 0.8 (SiO2; petroleum ether:EtOAc, 8:2). Td: 310 °C. 1H NMR (300 MHz, CDCl3): δ = 6.97 (d, 3J = 15.9 Hz, 2H), 7.04–7.20 (m, 26H), 7.27–7.32 (m, 12H), 7.47–7.55 (m, 6H), 7.85 (d, 3J = 15.9 Hz, 2H), 8.04 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 112.7, 122.6, 122.8, 3
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123.5, 123.7, 124.3, 124.6, 125.1, 125.2, 126.4, 128.7, 129.49, 129.54, 129.6, 130.3, 136.0, 137.2, 147.3, 147.5, 148.6, 148.9, 163.0, 164.9 ppm. IR (ATR): ν = 3034, 1588, 1558, 1489, 1273, 1174, 970, 750, 693 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C64H49N5 [M+] 887.3983, found 887.3982.
(E,E,E)-2-[(6-Methoxynaphtalen-2-yl)ethenyl]-4,6-bis(4-methoxystyryl)pyrimidine (2n).
Synthesized from 1e (56 mg, 0.19 mmol) and 4-methoxybenzaldehyde (52 mg, 0.38 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 77 mg (77 %); yellowish solid. Rf: 0.3 (SiO2; petroleum ether:EtOAc, 8:2). Td: 290 °C. 1H NMR (300 MHz, CDCl3): δ = 3.86 (s, 6H), 3.94 (s, 3H), 6.94–7.19 (m, 9H), 7.38 (d, 3J = 15.9 Hz, 1H), 7.59–7.61 (m, 4H), 7.74–
7.98 (m, 6H), 8.24 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 55.5, 106.1, 113.1, 114.5, 119.2, 124.5, 124.8, 127.4, 127.8, 128.6, 128.9, 129.1, 129.2, 130.0, 132.1, 135.1, 136.2, 137.9, 158.4, 160.7, 163.1, 164.7 ppm. IR (ATR): ν = 2935, 1601, 1559, 1505, 1366, 1246, 1169, 1027, 959, 838, 809 cm-1. HR-MALDI-MS (DCTB): m/z calculated for C35H30N2O3 [M+] 526.2251, found 526.2252.
(E,E,E)-2-[(6-Methoxynaphtalen-2-yl)ethenyl]-4,6-bis(4-
dimethylaminostyryl)pyrimidine (2o). Synthesized from 1e (53 mg, 0.18 mmol) and 4-N,N- dimethylaminobenzaldehyde (54 mg, 0.36 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization from CH2Cl2/n-heptane. Yield: 70 mg (70 %); orange solid. Rf: 0.2 (SiO2; petroleum ether:EtOAc, 8:2). Td: 250 °C. 1H NMR (300 MHz, CDCl3): δ = 3.03 (s, 12H), 3.94 (s, 3H), 6.72–6.75 (m, 4H), 6.92 (d, 3J = 15.9 Hz, 2H), 7.07–7.18 (m, 3H), 7.38 (d, 3J = 15.9 Hz, 1H), 7.54–7.57 (m, 4H), 7.74–7.90 (m, 5H), 7.99 (s, 1H), 8.23 (d, 3J = 15.9 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.4, 55.5, 106.1, 112.2, 112.3, 119.1, 122.0, 124.3, 124.8, 127.3, 128.2, 128.4, 129.2, 130.0, 132.2, 135.0, 136.7, 137.4, 151.2, 158.3, 163.4, 164.5 ppm All the atoms of 3
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